1. Field of the Invention
This invention relates in general to a switching power source device so as to produce a stable output voltage. Particularly, it relates to a switching power source which is suitable for supplying large amounts of electrical power.
2. Description of the Prior Art
A variety of switching power source devices are known in which the DC input power source turns off and on in a controlled manner so that the desired constant voltage output is obtained from the power source transformer. An example of this kind of switching power source device is described in Japanese Patent Application Nos. 215854/1984 and 211841/1984 filed by the assignee of the present application. Such switching power source device is designed to be used with the saturable reactor transformer, and the series resonance impedance in the primary of the power source transformer is controlled depending upon the output voltage from the secondary so as to control the exciting current for stabilizing the output voltage.
FIG. 12 illustrates an example of a prior art switching electrical power source device wherein the DC input power source to the power source device is obtained by, for example, rectifying and smoothing the output of a commercial AC input power source 101 using a full rectifier 102 in a bridge diode configuration and a smoothing capacitor 103. The output from the DC input source is supplied through a transistor Q1 and a primary winding N.sub.A of a converter drive transformer 111 to a series resonance circuit comprising a capacitor 112, a controlled winding N.sub.R of a saturable reactor transformer 113 which controls the power and a primary winding N.sub.1 of a power isolation transformer 114. The current of the input power source is turned off and on by a two transistor self-excited oscillating drive circuit 115 which includes a first switching transistor Q1 which has its base connected to a series resonance circuit consisting of a secondary winding N.sub.B1 of the converter drive transformer 111 and a capacitor C.sub.B1 and a second switching transistor Q2 which has its base connected to a series resonant circuit consisting of a secondary winding N.sub.B2 of the transformer 111 and a capacitor C.sub.B2.
The saturable reactor transformer 113 has the controlled winding N.sub.R and a control winding N.sub.C FIG. 13 illustrates the controlled winding N.sub.R wound so as to straddle two adjacent legs 113a and 113b of the four magnetic legs 113a through 113d of a magnetic core 113e. The control winding N.sub.C is wound so as to straddle, for example, the magnetic legs 113b and 113c and in a direction such that the long axis of the winding is orthogonal to the long axis of the winding N.sub.R. The magnetic fluxes through the saturable reactor transformer 113 are controlled depending upon the control current supplied to the control winding N.sub.C so as to control magnetic fluxes through the transformer 113 and, thus, the inductance of the control winding N.sub.R.
A parallel resonant capacitor C.sub.S and a rectifying smoothing circuit 116 are connected to the secondary winding N.sub.2 of a power isolation transformer 114. The DC output voltage from circuit 116 is converted by a control circuit 117 into a control current which is supplied to the control winding N.sub.C of the saturable reactor transformer 113.
Thus, the inductance of the saturable reactor transformer 113 changes depending upon fluctuations in the DC output voltage and this changes the series resonance impedance in the primary winding of the power isolation transformer 114 so as to cause changes in the exciting current to control the DC output voltage to a constant value.
The switching power source device described has the disadvantages in that heat radiation from the saturable reactor transformer 113 makes it necessary as shown in FIG. 13 to provide a heat radiation plate 113f to the magnetic core 113e using a Ushaped bracket 113g which clamps the plate 113f to the shield case of the power source so as to provide heat radiation. Also, a large magnetic flux leakage occurs from the saturable reactor 113. The switching power source device has an AC-DC conversion efficiency which can reach an efficiency of 83-85% at the highest. It is desired to obtain higher conversion efficiency for switching power sources particularly when the load power exceeds approximately 100 watts.